Congestive Heart Failure (CHF) is a form of heart disease still increasing in frequency. According to the American Heart Association, CHF is the “Disease of the Next Millennium”. The number of patients with CHF is expected to grow even more significantly as an increasing number of the “Baby Boomers” reach 50 years of age.
CHF is a condition that occurs when the heart becomes damaged and reduces blood flow to the organs of the body. If blood flow decreases sufficiently, kidney function becomes impaired and results in fluid retention, abnormal hormone secretions and increased constriction of blood vessels. These results increase the workload of the heart and further decrease the heart's pumping ability and, in turn, causes further reductions in blood flow to the kidney. It is believed that the progressively-decreasing perfusion of the kidney is a principal non-cardiac cause perpetuating the downward spiral of the “Vicious Cycle of CHF”. Moreover, the fluid overload and associated clinical symptoms resulting from these physiologic changes are a predominant cause for excessive hospital admissions, poor quality of life and large costs to the health care system due to CHF.
There is a long-felt demand for a miniature and portable extracorporeal fluid treatment devices for patients suffering from repeated fluid overload. Such a device might be worn during the day as the patient moves about. The device would preferably be easy to use. If the procedure for inserting catheters percutaneously is too complicated, it will be difficult to have sufficiently trained clinical personnel available to insert the catheter. Standard OTN (Over The Needle) catheters generally require nurses with intravenous (IV) insertion experience to insert catheters to gain blood access for the fluid removal device. When midline catheters are used for accessing blood peripherally, the insertion of the catheter is limited to clinicians with the required training. To insert PICC (Peripherally Inserted Central Catheter) line requires IV nurses with training in seldinger insertion technique and doctors with similar training.
Utilizing an ultrafiltration fluid removal device that uses standard IV access would greatly simplify the process. A simpler approach would be to use an implanted port whereby the nurse could quickly gain access to blood flow. Implanted ports are commonly used for drug infusion therapies and are ideal for long term access. Implanted ports are also accessed via IV needles. Ports have reduced infection rates when compared with standard central venous percutaneous catheters and are ideal for repeatable access over long periods of time.
Peripheral IV blood access is not without its own inherent issues, which include: (1) The blood flow may be limited and intermittent with peripheral access because the further down the peripheral vein tree blood flow is accessed there is less blood flow is available and the more that the available blood flow is subject to fluctuations in flow. Implanted ports can overcome many of these limitations because they are placed centrally. (2) The majority of nurses are comfortable using OTNs with sizes of 20 G and less. These are simpler to insert, cause less trauma and facilitate multiple insertions. If larger catheters gauge sizes are required, the percentage of nurses that can gain access without trouble quickly diminishes which would cause a further hurdle to the usage of such an ultrafiltration device. However, the smaller the gauge size of the catheter the smaller its internal diameter which results in a limitation in the maximum blood flow that can be achieved. This limitation is due to the maximum positive and negative pressures that blood can safely be exposed to without compromising patient safety. Patients with fluid overload also suffer from peripheral edema and the edema tends to hide the veins by placing them further from the skin surface making it more difficult to locate them. In these cases the clinician resorts to multiple sticks before a vein is located. The smaller the IV catheter the more acceptable such a procedure is. Larger gauge needles increase the trauma to the arm and are less acceptable to patients. Implanted ports will overcome many of these limitations because access is gained via a septum which when located makes gain blood flow access relatively simple.
When low blood flow is used in an extracorporeal device, it creates a significant engineering challenge. In general the lower the blood flow, the longer the residence time of blood extracorporeally and the greater the propensity of the device to clot. Lower blood flows increases the difficulty of pressure sensors to reliably detect infusion disconnects. Lower blood flows yield lower pressure drops that result in a large pressure sensor signal to noise ratio between the pressure drop attributed to the access and variations in pressure due to patient movement. When peripheral vein access is used or when a patient is treated for an extended period of time, it is normal to expect the patient to be mobile and that the patient will move about, bend over and lift their arms to stretch occasionally during treatment. Every centimeter (cm) in height change of the patient's arm having the withdrawal and/or infusion catheter results in a 0.75 mmHg pressure increase or decrease in the blood circuit due to the resultant change in static head pressure. Accordingly, the ratio between variation in static head pressure due to patient movement and needle pressure drop due to blood flow should be relatively insignificant so that false alarms will not be annunciated. Disconnection of blood tubes/access in the circuit are generally detected by a reduction in the pressure drop across a needle. It may be hard to distinguish pressure fluctuations due to patient motion and to needle disconnects. It is accepted medically that a blood loss of 100 ml or less will not result in the patients' health being compromised. In the case of the typical dialysis machine this time period is 100 ml/400 ml/min or 15 seconds. Disconnection algorithms have to be extremely sensitive to ensure patient safety. In hemodialysis standards, such as IEC 60601-2-16, the standards committee was aware that there are inherent limitations in using pressure measurements for detecting disconnects which is why devices which use high blood flows have been limited to ICU and dialysis clinic use up until recently. It is also recognized as being an increased risk for the use of home dialysis.